The invention relates to an apparatus for the machine recognition of optical-diffraction markings of the kind set forth in the classifying portion of claims 1 and 4.
Such apparatuses for the machine recognition of optical-diffraction markings are suitable for checking the authenticity of documents, postage stamps, bank notes, identity papers, passes and the like, having a security feature which contains corresponding optical-diffraction markings.
German laid-open application (DE-OS) No 23 52 366 illustrates the basic arrangement of a reading device for a transparent security structure utilising Fourier transformation by a suitable optical arrangement having a lens. Parallel light which is incident in perpendicular relationship on a macroscopic security structure formed with prisms is deflected by refraction. Identical prisms involving the same azimuthal orientation deflect the light in the form of parallel beams and are focused by the optical arrangement onto a point in the focal plane of the optical arrangement. The distribution of the levels of light intensity in the focal plane forms the recognition feature.
The reading device known from German laid-open application (DE-OS) No 25 38 875 utilises Fourier transformation to recognise a predetermined relief structure of a uniform diffraction grating, by light irradiation. The reading device compares the levels of intensity of the light beams which are deflected in diffraction orders of different magnitudes, and it is thus possible to infer the cross-sectional shape of the relief used in the diffraction grating. An increase in the degree of security in relation to authenticity recognition is achieved by sequential reading of successive diffraction gratings or by means of a plurality of reading devices which investigate diffraction gratings disposed on the substrate at various predetermined locations in parallel relationship.
Swiss patent specification No 653 160 describes a development of the reading device shown in DE-OS No 25 38 875 for an authenticity feature which includes a plurality of simultaneously illuminated portions with different diffraction structures as encoding means. The light which is diffracted at the diffraction structures is focused by an optical means in the focal plane thereof. In the focal plane the locations of high light intensity are determined only by the parameters such as spatial frequency, azimuth etc of the various diffraction structures. If those locations coincide with those of light receivers arranged in a predetermined fashion, the authenticity feature is recognised as such.
U.S. Pat. Nos. 4,034,211 and 4,023,010 describe a further reading device for sequential optical scanning of a data track comprising successive equal-sized diffraction gratings with a thin light beam which illuminates only a portion of the diffraction grating. These publications refer to the errors which are possible in a practical context when effecting optical scanning such as for example tilting of the plane of the diffraction grating relative to the reading plane and a larger or smaller spacing of the plane of the diffraction grating relative to the reading plane. The known optical arrangement for Fourier transformation eliminates the effects of spacing errors.
EP-0 718 834 A2 describes an optical data carrier with a high recording density (bits/cm2) and a reading device for securely reading the sequential series on the data carrier of information patterns which are composed of a plurality of diffraction gratings.
The reading device known from EP 0 533 448 A2 has symmetrical large-area photodetectors which are arranged around the reading beam and whose light-sensitive surfaces cover the sectors of a circular ring or two concentric circular rings. The reading device recognises sequential series, contained on a data carrier, of information patterns which are composed of a plurality of diffraction gratings.
All those reading devices require precise guidance of the data carrier while precisely maintaining the spacing and accurate orientation of the diffraction patterns on the data carrier, in order in any way to permit the diffraction pattern to be read off. In the case of the previously known reading devices, it is not possible to exclude the reading device being easily deceived by means of suitably arranged mirror or prism surfaces.
The reading device in accordance with Swiss patent specification 653 162 excludes such a deception effect insofar as the change in the diffraction angle is detected upon a periodic change in the wavelength of the light for irradiating the diffraction grating.
A security feature which can be copied only with a very great amount of difficulty and the production thereof from surface elements with reflecting diffraction gratings are described in EP-105 099, EP-169 326 and U.S. Pat. No. 5,032,003. The diffraction gratings reflect incident white light, divided up into colours, into the various diffraction orders, at given azimuth angles, wherein the grating parameters of the diffraction gratings such as profile shape, spatial frequency and azimuth direction are crucial in terms of the diffraction characteristics. The use of asymmetrical profile shapes for the inconspicuous incorporation of machine-readable information in the pattern of such security elements is taught by EP-360 969 A1 and EP-366 858 A1 which also show diagrammatic embodiments of reading devices which can recognise asymmetry of the profile shapes.
The object of the present invention is to provide an inexpensive, simple, optical reading device for a data carrier with an optically diffractive security feature, whose optical features can be read out by machine simultaneously and without high demands in terms of the orientation of the data carrier.
In accordance with the invention that object is attained by the features of claims 1 and 4. Advantageous configurations are set forth in the appendant claims.